The COCOMO cost estimation model is used by thousands of software project
managers, and is based on a study of hundreds of software projects. Unlike other cost
estimation models, COCOMO is an open model, so all of the details are published,
including:

The underlying cost estimation equations

Every assumption made in the model (e.g. "the project will enjoy good
management")

Every definition (e.g. the precise definition of the Product Design phase of a
project)

The costs included in an estimate are explicitly stated (e.g. project managers are
included, secretaries aren't)

Because COCOMO is well defined, and because it doesn't rely upon proprietary
estimation algorithms, SystemStar offers these advantages to its users:

COCOMO estimates are more objective and repeatable than estimates made by
methods relying on proprietary models

COCOMO can be calibrated to reflect your software development environment,
and to produce more accurate estimates

SystemStar is a faithful implementation of the COCOMO model that is easy to use on small
projects, and yet powerful enough to plan and control large projects.

Typically, you'll start with only a rough description of the software system that you'll be
developing, and you'll use SystemStar to give you early estimates about the proper schedule
and staffing levels. As you refine your knowledge of the problem, and as you design
more of the system, you can use SystemStar to produce more and more refined estimates.

SystemStar allows you to define a software structure to meet your needs. Your initial
estimate might be made on the basis of a system containing 3,000 lines of code. Your
second estimate might be more refined so that you now understand that your system will
consist of two subsystems (and you'll have a more accurate idea about how many lines of
code will be in each of the subsystems). Your next estimate will continue the process --
you can use SystemStar to define the components of each subsystem. SystemStar permits you to
continue this process until you arrive at the level of detail that suits your needs.

One word of warning: It is so easy to use
SystemStar to make software cost estimates, that
it's possible to misuse it -- every SystemStar user should spend the time to learn the
underlying COCOMO assumptions and definitions from Software Engineering
Economics and Software Cost Estimation with COCOMO II.

Introduction to the COCOMO Model

The most fundamental calculation in the COCOMO model is the use of the Effort
Equation to estimate the number of Person-Months required to develop a project. Most of
the other COCOMO results, including the estimates for Requirements and
Maintenance, are derived from this quantity.

The COCOMO calculations are based on your estimates of a project's size in
Source Lines of Code (SLOC). SLOC is defined such that:

Only Source lines that are DELIVERED as part of the product are included -- test
drivers and other support software is excluded

SOURCE lines are created by the project staff -- code created by applications
generators is excluded

One SLOC is one logical line of code

Declarations are counted as SLOC

Comments are not counted as SLOC

The original COCOMO 81 model was defined in terms of Delivered Source
Instructions, which are very similar to SLOC. The major difference between
DSI and SLOC is that a single Source Line of Code may be several physical
lines. For example, an "if-then-else" statement would be counted
as one SLOC, but might be counted as several DSI.

In the COCOMO II model, some of the most important factors contributing to a project's
duration and cost are the Scale Drivers. You set each Scale Driver to describe
your project; these Scale Drivers determine the exponent used in the Effort
Equation.

The 5 Scale Drivers are:

Precedentedness

Development Flexibility

Architecture / Risk Resolution

Team Cohesion

Process Maturity

Note that the Scale Drivers have replaced the Development Mode of
COCOMO 81. The first two Scale Drivers, Precedentedness and Development
Flexibility actually describe much the same influences that the original
Development Mode did.

Cost Drivers

COCOMO II has 17 cost drivers -- you assess your project, development environment, and team to set each cost driver. The cost drivers are multiplicative factors that determine the effort required to complete your software project. For example, if your project will develop software that controls an airplane's flight, you would set the Required Software Reliability (RELY) cost driver to Very High. That rating corresponds to an effort multiplier of 1.26, meaning that your project will require 26% more effort than a typical software project.

Where
EAF Is the Effort Adjustment Factor derived from the Cost Drivers
E Is an exponent derived from the five Scale Drivers

As an example, a project with all Nominal Cost Drivers and Scale Drivers would have an EAF of 1.00 and exponent, E, of 1.0997. Assuming that the project is projected to consist of 8,000 source lines of code, COCOMO II estimates that 28.9 Person-Months of effort is required to complete it:

Effort = 2.94 * (1.0) * (8)1.0997 = 28.9 Person-Months

Effort Adjustment Factor

The Effort Adjustment Factor in the effort equation is simply the product of the effort multipliers corresponding to each of the cost drivers for your project.

For example, if your project is rated Very High for Complexity (effort multiplier of 1.34), and Low for Language & Tools Experience (effort multiplier of 1.09), and all of the other cost drivers are rated to be Nominal (effort multiplier of 1.00), the EAF is the product of 1.34 and 1.09.

Effort Adjustment Factor = EAF = 1.34 * 1.09 = 1.46

Effort = 2.94 * (1.46) * (8)1.0997 = 42.3 Person-Months

COCOMO II Schedule Equation

The COCOMO II schedule equation predicts the number of months required to complete your software project. The duration of a project is based on the effort predicted by the effort equation:

Duration = 3.67 * (Effort)SE

Where
Effort Is the effort from the COCOMO II effort equation
SE Is the schedule equation exponent derived from the five Scale Drivers

Continuing the example, and substituting the exponent of 0.3179 that is calculated from the scale drivers, yields an estimate of just over a year, and an average staffing of between 3 and 4 people:

Duration = 3.67 * (42.3)0.3179 = 12.1 months

Average staffing = (42.3 Person-Months) / (12.1 Months) = 3.5 people

The SCED Cost Driver

The COCOMO cost driver for Required Development Schedule (SCED) is unique, and requires a special explanation.

The SCED cost driver is used to account for the observation that a project developed on an accelerated schedule will require more effort than a project developed on its optimum schedule. A SCED rating of Very Low corresponds to an Effort Multiplier of 1.43 (in the COCOMO II.2000 model) and means that you intend to finish your project in 75% of the optimum schedule (as determined by a previous COCOMO estimate). Continuing the example used earlier, but assuming that SCED has a rating of Very Low, COCOMO produces these estimates:

Duration = 75% * 12.1 Months = 9.1 Months

Effort Adjustment Factor = EAF = 1.34 * 1.09 * 1.43 = 2.09

Effort = 2.94 * (2.09) * (8)1.0997 = 60.4 Person-Months

Average staffing = (60.4 Person-Months) / (9.1 Months) = 6.7 people

Notice that the calculation of duration isn't based directly on the effort (number of Person-Months)
-- instead it's based on the schedule that would have been required for the project assuming it had been developed on the nominal schedule. Remember that the SCED cost driver means "accelerated from the nominal schedule".

The SystemStar command Constraints | Constrain Project displays a dialog box that lets you trade off duration vs. effort (SCED is set for you automatically).
You can use the dialog box to constrain your project to have a fixed duration,
or a fixed cost.